This invention relates to a method for producing a phosphor, and more particularly a method for producing a phosphor using carbon particles of a nanostructure and a phosphor produced by the method.
A method for producing a phosphor using ultrafine particulate crystals (hereinafter referred to as "nanocrystals") of oxides activated with rare earth elements is disclosed in U.S. Pat. No. 5,637,258. The nanocrystal oxides disclosed are produced according to a method similar to sol-gel techniques. The method is executed at a room temperature using a solution of Y--Eu n-butoxide [Y--Eu(On-Bu)x] in butanol (C.sub.4 H.sub.9 OH).
An n-butoxide solution of yttrium (Y) and Eu acting as an activator element is prepared as follows:
Synthesis of Y n-butoxide is carried out by preparing Na iso-propoxide via a reaction between metal Na and iso-propanol and adding YCl.sub.3 to the Na iso-propoxide to obtain Y iso-propoxide. EQU YCl.sub.3 +3NaOi-Pr.fwdarw.Y(Oi-Pr).sub.3 +3NaCl
Then, n-butanol is added to the solution thus obtained, to thereby replace the iso-propyl group with an n-butyl group, followed by azeotropic distillation, resulting in obtaining Y n-butoxide. EQU Y(Oi-Pr).sub.3 +n-BuOH.fwdarw.Y(On-Bu).sub.3 +i-PrOH
Synthesis of n-butoxide of an activator such as Eu or the like may be carried out in a manner similar to that of Y butoxide.
Mixing between n-butoxide of Y synthesized and that of the activator Eu at a temperature of 117.degree. C. leads to an n-butoxide solution of Eu acting as an activator element and Y.
Now, conventional synthesis of a phosphor constituted by nanocrystals of oxides activated with rare earth elements which is carried out using the thus-prepared solution of Y--Eu n-butoxide [Y--Eu(On-Bu)x] in butanol (C.sub.4 H.sub.9 OH) will be described hereinafter.
Acetic acid is added to the Y--Eu n-butoxide/butanol solution, leading to acetylation of the solution, as follows: EQU Y--Eu(On-Bu)x+CH.sub.3 COOH.fwdarw.Y--Eu(O--COCH.sub.3)x+n-BuOH
Both are mixed together and allowed to stand for 0.5 to 1.0 hour, during which the reaction takes place, resulting in the Y--Eu acetate and n-butanol being produced. The pH is .about.8.0, which is alkaline. Addition of acetic acid thereto causes the pH to be decreased to .about.5.0.
Then, water is added to the solution, leading to hydrolysis, resulting in hydroxide of Y--Eu being formed. EQU Y--Eu(O--COCH.sub.3)x+n-BuOH +H.sub.2 O.fwdarw.Y--Eu(OH).sub.3 +OH
Then, an aqueous NaOH solution is added to the solution containing the hydroxide of Y--Eu at 85.degree. C. to adjust the pH at 13.5, resulting in nanocrystals of yttrium oxide activated with Eu (Y.sub.2 O.sub.3 :Eu) being precipitated.
The thus-precipitated nanocrystal particles are then washed with water and acetone, to thereby remove any organic by-products therefrom. The particles are dried, followed by washing with a dilute NaOH solution.
As will be noted from the above, the conventional method for manufacturing the nanocrystal oxide phosphor includes a variety of reaction steps using various kinds of solvents, so that a surface of the particles of the phosphor ultimately obtained is polluted by the organic by-products. This causes the pollutant to absorb luminous energy on the surface of the particles of the phosphor, resulting in luminescence of the phosphor being restrained. Such a phenomenon is called non-luminous relaxation. In order to avoid such a problem, it would be considered to carry out calcination of the phosphor for removal of the by-products. However, the calcination causes the by-products heated to intrude between the fine particles of the phosphor, leading to a failure in complete removal of the by-products.
Also, the conventional method fails to control a particle size distribution of the phosphor particles produced, leading to spreading of a luminous band of the phosphor.